Various areas of physics require spatially separating radiant energy into its spectral components such as by frequency and/or polarization. By way of example such fields include solar cells, image array sensors, filters, energy harvesting devices, certain types of reflectors, and the like. Similarly, various areas will benefit from mixing various spectral components into a broader type of radiant energy, combining a plurality of ‘narrower’ spectral components into a ‘broader’ radiant energy.
In its most basic form, the term ‘refraction’ means the change of direction of a ray of light, sound, heat, radio waves, and other forms of wave energy, as it passes from one medium to another. Generally waves of different frequencies would refract at different angles and thus refraction tends to spatially separate multispectral radiation into its spectral components by frequency. The term ‘spectral component’ will relate to the energy or a portion thereof in the spectral range of interest, which is characterized by its frequency, polarization, phase, flux, intensity, incidence, radiosity, energy density, radiance, or a combination thereof. Multi-spectral energy relates to energy having at least two spectral components.
Electromagnetic (EM) radiant energy extends over a broad frequency spectrum, however many applications deal only with portions of this spectrum. Light is one form of radiant energy which may be considered as an alternating EM radiation at very high frequency. Humans perceive different light frequencies as different colors, but there is a large amount of radiation that is not perceived by humans, generally known as UV (Ultra Violet), and IR (Infra Red), and the term light will be extended thereto. Visible light ranges generally between 760-300 nm and roughly corresponds to the peak intensity of solar radiation transmitted through the atmosphere. Infrared radiation ranges from the extreme far end of 1 mm (33 THz; millimeter radio waves) to about 760 nm. The range of millimeter waves, also known as Extra High Frequency (EHF), is specifically considered as part of the possible spectral range of different embodiments, as their behavior is sufficiently similar for the purposes of combining and separating radiant energy, so as to benefit from various aspects of the invention. As the human eye is capable of directly sensing and differentiating between light of different frequencies, it will be used oftentimes to explain the operation of different aspects of the invention for the sake of brevity and increased clarity, however the spectral range of interest to which those examples relate may be larger, and depends on the application at hand. With changes in dimensions, materials and the like, the principle described herein extend to any electromagnetic radiant energy and thus the all or portions of the spectrum ranging from the EHF to UV should be considered equivalent, unless otherwise specified or clear from the context.
It is seen therefore that radiant energy extends over a very broad radiation spectrum, and many applications would need to cover only portions of this spectrum. By way of example, for solar energy applications the spectral range of interest will likely be a spectrum containing most if not all of the solar spectrum available at the location where the solar cell is to be deployed, or the portion thereof which is economically used by the device at hand, typically of wavelength within 2-3 μm to 300 nm for example. The spectral range of interest for most display devices will fall within the visible light spectrum, even if some special application demand extending the spectral range. In some applications a specific wavelength may be desirably attenuated, such as by way of example reduction of blue light for pilot related devices. Yet, for devices directed to heat energy recovery, it is likely that only the infra-red portion of the spectral range is of interest. Similarly, the spectral range of interest may be applicable to portions of a device, such that by way of example, a device may be directed to a broad spectrum, but portions thereof may be directed to a narrower spectrum, and the spectral range of interest is thus limited to the range of interest for that portion of the device. By way of a non-limiting example a television may occupy a display portion that utilizes CRTR's as described below and additional emissions such as audio outputs. The spectral range of interest of the display may only extend to the visible range, even if the device as a whole includes the aural range as well, the aural range does not fall within the spectral range of the CRTR used in the television. It is seen therefore that the application at hand determines the spectral range of interest, and that a spectral range of interest may differ by application, an apparatus, or a portion thereof. Regarding lateral waveguides, which is described below, each waveguide may have its own spectrum of interest, which may differ from the spectral range of interest of an adjacent waveguide. Similarly, for array of CRTRs, each CRTR may have its own spectral range or ranges of interest.
Therefore, the spectral range of interest is defined herein as relating to any portion or portions of the total available spectrum of frequencies and/or polarizations, which is being utilized by the application, apparatus, and/or portion thereof, at hand, and which is desired to be filtered, channeled, detected, emitted, and/or reflected utilizing the technologies, apparatuses, and/or methods of the invention(s) described herein, or their equivalents.
At sufficiently high frequencies, radiant energy is also commonly considered as a flow of photons, which are quantized units of energy which increases with frequency. Under this quantum physics description, the energy density associated with electric and magnetic fields are probability distributions of photons. Therefore certain terms that are common to simple electromagnetic energy can be better clarified as relating to the spectrum of interest. Thus, a dielectric material in the above mentioned energy spectrum of interest relates to a material having low conductivity, and having a band-gap between a filled valence band and an empty conduction band exceeding the energy of any photon in the spectrum of interest to a specific application. In contrast, a transparent conductor is a material having a finite but meaningful conductivity due to a partially filled conduction band or partially empty valence band but having a band-gap between the valence band and conduction band exceeding the energy of any photon in the spectrum of interest. These materials act like a dielectric at high frequencies but act like a conductor at low frequencies. Transparent dielectric materials also have low optical losses such that photons efficiently transmit through such material, at least at the spectrum of interest or a significant portion thereof.
While transparent conductors may be considered as wide bandgap semiconducting materials, they are used as conductors in most applications. Dielectrics, transparent conductors, and semiconductors, as used in these specifications, refer to materials that have a dielectric constant at optical frequencies; however the distinction between a semiconductor and the remaining materials is that the bandgap of a semiconductor is not substantially larger than the photon energy. As a general and non-limiting guideline, table 1 describes several characteristics of the different conductive, insulating, and semi-conductive materials.
TABLE 1TransparentMaterialMetalconductorSemiconductorDielectricBandgap→ 0>>photon≤photon>>photonDC ConductivityhighgoodVaries→ 0Optical PropertyreflectivetransparentabsorptivetransparentDielectric constantcomplexlow losslossylow loss
Waveguides are a known structure for trapping and guiding electromagnetic energy along a predetermined path. An efficient waveguide may be formed by locating a layer of dielectric or semiconducting material between cladding layers on opposite sides thereof, or surrounding it. The cladding may comprise dielectric material or conductors, commonly metal. Waveguides have a cutoff frequency, which is dictated by the wave propagation velocity in the waveguide materials, and the waveguide width. As the frequency of the energy propagating in the waveguide approaches the cutoff frequency Fc, the energy propagation speed along the waveguide is slowed down. The energy propagation of a wave along a waveguide may be considered as having an angle relative to cladding. This angle is determined by the relationship between the wavelength of the wave and the waveguide width in the dimension in which the wave is being guided. If the width of the waveguide equals one half of the wave wavelength, the wave reaches resonance, and the energy propagation along the waveguide propagation axis stops.
In these specifications, the term cladding penetration state relates to a condition where energy confined by the tapered core waveguide leaves the waveguide via the cladding. Generally each waveguide has some negligible penetration of energy into the cladding, however cladding penetration state occurs when a significant amount of energy is transported through the cladding. Cladding penetration state is generally frequency related, and energy of one frequency may reach cladding penetration state at a different set of conditions than the cladding penetration state of another frequency. By way of non-limiting example, if 66% of the energy of frequencies between F1 and F2 will exit a hypothetical waveguide via the cladding at a distance between 1 um to 2 um from the waveguide aperture, the cladding penetration state for F1-F2 would exist between 1-2 um from the aperture. Other frequencies may or may not overlap such range partially or completely. Notably the number 66% has been arbitrarily selected by way of example only, and may be modified as an engineering choice according to the application at hand.
In these specifications, cladding penetration state is used primarily to define a location or a region where cladding penetration would occur, rather than necessarily the actual occurrence of cladding penetration. As discussed below, energy may be coupled into the waveguide core via the cladding at the region about which cladding penetration state would occur, as well as be outputted therefrom.
Stationary resonance condition is a condition in a waveguide where the local cutoff frequency of the waveguide equals the frequency of a wave guided by the waveguide, such that the guided wave reflects repeatedly between opposing surfaces of the guide, however the corresponding component of energy velocity along the waveguide propagation axis is zero. As the wave frequency approaches the local cutoff frequency of the waveguide, a sharp decrease in the wave propagation (group) velocity is noticed at the immediate vicinity of the cutoff dimension, as may be seen by way of example in the lower graph of FIG. 3. While complete stationary resonance condition is seldom if ever achievable, for the purpose of these specifications a stationary resonance (SRC) condition will be considered a situation where the guided wave is sufficiently close to the complete stationary resonance condition to significantly lower than the speed of light in the bulk material of the waveguide. Stated differently, when a wave falls within the zone of the sharp decrease in velocity it is considered to be in SRC.
With proper selection of cladding material and dimensions, energy will reach a cladding penetration state and depart the waveguide through the cladding at this stationary resonant condition. This mechanism is related to by the acronym CPS-SRC. CPS-SRC often occurs with reflective cladding, comprising thin metallic cladding. Notably a metallic cladding of lower thickness than the penetration depth to which the cladding is locally exposed would allow energy to pass therethrough and such cladding may be utilized. Furthermore, when certain metals are disposed at low thicknesses they tend to “ball-up” and form small “islands”. Such “balled-up” metal, and/or intentionally perforated metal cladding may also form a discontinuous metal film cladding in a reflective CRTR waveguide.
Total internal reflection (TIR) is a phenomenon which occurs when a guided wave hits the boundary between the core and the cladding below a certain angle relative to the local propagation axis of the waveguide. The angle is known as the critical angel of total Internal Reflection. When a guided wave reaches or exceeds the critical angle it departs the waveguide via the cladding under normal refraction. Slightly below this critical angle the internal reflection by a finite cladding becomes incomplete in a process known as Frustrated Total internal Reflection (FTIR). This condition occurs mostly with dielectric cladding, but metallic claddings with small perforations or with thicknesses at or near the tunnel distance also have angle dependent reflection coefficients, resulting in a situation analogous to FTIR. Cladding penetration condition reached by a wave exceeding the critical angle of total internal reflection is referred to hereinafter as CPS-FTIR. Both CPS-FTIR and CPS-SRC are characterized by energy traversing the cladding, thus CPS, or ‘cladding penetration state’ will be used interchangeably to denote CPS occurring through any mechanism.
Collectively, objects, materials, and structures, which inter-convert electromagnetic and electrical energy are known by various names which denote equivalent structures, such as converters, transducers, absorbers, detectors, sensors, and the like. To increase clarity, such structures will be referred to hereinunder as ‘transducers’. By way of non-limiting examples, the term “transducer” relates to light sources, light emitters, light modulators, light sensors, photovoltaic materials including organic and inorganic transducers, quantum dots, CCD and CMOS structures, LEDs, OLEDs, LCDs, laser sources, receiving and/or transmitting antennas and/or rectennas, phototransistors photodiodes, diodes, electroluminescent devices, fluorescent devices, gas discharge devices, electrochemical transducers, and the like. Certain transducers may be configured to convert energy forms bidirectioanlly, such as a single transducer which may operate as a converter from electrical energy to radiant energy, and vice versa. Alternatively transducers may be built to convert only from one energy form to another. Transducers for conversion of radiant energy to electricity or electrical signals (hereinafter “LE”), or conversion of electrical signals into radiant energy such as light (hereinafter “EL”) are known.
A transducer of special construction is the RL type transducer, which is a reflective transducer. Reflective transducers controllably reflect radiant energy. Such transducers may comprise micro-mirrors, light gates, Liquid Crystals (LCD), and the like, positioned to selectively block the passage of radiant energy, and reflect it into a predetermined path, which is often but not always, the general direction the energy arrived from. Certain arrangements of semiconductor and magnetic arrangements may act as RL transducers by virtue of imparting changes in propagation direction of the radiant energy, and thus magnetic forces or electrical fields may bend a radiant frequency beam to the point that in effect, it may be considered as reflected. RL transducers may be fixed, or may be used to modulate the energy direction over time. Passive transducers such as LCD and micromirrors fall into the RL class of transducers when used to reflect incoming energy, but when used in conjunction with at least one light source, such transducers may also be considered as LE type transducers.
Radiant energy transducers, and especially LE transducers, typically employ normal incidence of radiant electromagnetic energy onto a conversion structure. Normal incidence has the limitation of a finite probability of detecting energy before it is transmitted through the conversion layer. Energy transmitted through the conversion layer is, at best, lost and, at worst, converted to heat in the supporting substrate. Several attempts has been made to provide transducers that use ‘side illumination’ in which the light is inserted from the side of the junction. Such examples include, inter-alia, in U.S. Pat. No. 3,422,527 to Gault, U.S. Pat. No. 3,433,677 to Robinson, and U.S. Pat. No. 4,332,973 to Sater.
Prisms and other refractive devices can be used to improve incidence angles, and to direct different frequencies of radiant energy to different regions of a transducer, where each region is optimized for a target frequency. U.S. Pat. No. 7,888,589 to Mastromattteo and U.S. Pat. No. 8,188,366 to Hecht, disclose examples of such devices. Different arrangements of concentrators are also known, which are operative to concentrate energy to transducers. U.S. Pat. No. 5,578,140 to Yogev et al. as well as Hecht provide examples to such arrangements. Those methods require significantly increased device area, and reduce the total energy per unit area (and per unit manufacturing cost) in exchange for increased efficiency.
Vertical optical waveguides are known in the prior art. U.S. Pat. No. 4,251,679 to Zwan depicts a plurality of transducing cavities having an inwardly inclined wall to receive impinging radiation. Two potential barrier strips each having different conduction electron densities; each potential barrier strip is connected to a conductor having a preselected conduction electron density whereby radiation impinging on a cavity will induce current flow which will be rectified across the potential barriers. U.S. Pat. No. 3,310,439 to Seney relates to embedding spaced dimensioned crystals into p-n semiconductor layers of a solar cell device. The crystals function as waveguides into the photovoltaic layer.
Tapered waveguide directed at trapping radiant energy, as opposed to emitting energy via the cladding, have been disclosed by Min Seok Jang and Harry Atwater in “Plasmionic Rainbow Trapping Structures for Light localization and Spectrum Splitting” (Physical Review Letters, RPL 107, 207401 (2011), 11 Nov. 2011, American Physical Society©). The article “Visible-band dispersion by a tapered air-core Bragg waveguide”, (B. Drobot, A. Melnyk, M. Zhang, T. W. Allen, and R. G. DeCorby, 8 Oct. 2012/Vol. 20, No. 21/OPTICS EXPRESS 23906, ©2012 Optical Society of America_ “Visible-band dispersion by a tapered air-core Bragg waveguide” B. Drobot, A. Melnyk, M. Zhang, T. W. Allen, and R. G. DeCorby, 8 Oct. 2012/Vol. 20, No. 21/OPTICS EXPRESS 23906, ©2012 Optical Society of America) describes out-coupling of visible band light from a tapered hollow waveguide with TiO2/SiO2 Bragg mirrors. The mirrors exhibit an omnidirectional band for TE-polarized modes in the ˜490 to 570 nm wavelength range, resulting in near-vertical radiation at mode cutoff positions. Since cutoff is wavelength-dependent, white light is spatially dispersed by the taper. These tapers can potentially form the basis for compact micro-spectrometers in lab-on-a-chip and optofluidic micro-systems. Notably, Bragg mirrors are very frequency selective, complex to manufacture, and require at least a width higher than ¾ wavelength to provide any breadth of spectrum. In addition to the very narrow band, the Bragg mirrors dictate a narrow bandwidth with specific polarization, while providing however a fine spectral resolution.
However the known art does not provide a multi purpose small scale splitter/combiner/reflector of radiant energy. There is therefore a clear and heretofore unmet need for a small-scale spectral manipulation structure that would do one or more of: split multispectral electromagnetic radiation to obtain spectral component(s) contained in the multispectral radiation; mix spectral components to obtain multispectral electromagnetic radiation; redirect incoming electromagnetic energy so as to be diverted at some nonzero angle from its initial propagation direction; separate electromagnetic components by polarization; combine electromagnetic components of different polarizations, controllably reflect certain spectral components, and any combination of the above.